game802

var markerPos = $('#marker').offset().top,
    output = $('output');

$(window).on('scroll', function() {
    output.text($(this).scrollTop() >= markerPos ? $(this).scrollTop() - markerPos : 0);
});

<a href="#" class="btn" data-active="block-item-1">Блок #1</a>
<a href="#" class="btn" data-active="block-item-2">Блок #2</a>
<a href="#" class="btn" data-active="block-item-3">Блок #3</a>

<div class="block-item block-item-1">
	Текст блока #1
</div>

<div class="block-item block-item-2">
	Текст блока #2
</div>

<div class="block-item block-item-3">
	Текст блока #3
</div>

$('.btn').click(function(event) {
	event.preventDefault();

	var blocks = $('.block-item'), 
		active = $(this).data('active');

	blocks.fadeOut(0);
	$('.block-item.'+active).fadeIn(400);
});